The present invention relates to anesthesia patient breathing circuits and, more particularly, to a component of a breathing circuit that allows the relief of a selected maximum pressure in the breathing system.
Patient breathing circuits are utilized to convey gasses containing an anesthetic vapor to a patient to carry out the anesthetizing of that patient. In general, there are a variety of circuits and one particular circuit may be selected for use depending on the particular operation or upon the preference of the anesthesiologist. The overall purpose, however, is to convey those gases from an anesthesia machine where the proper combination of gases are mixed to deliver that mixed gas to the patient.
Accordingly, the components of such patent breathing circuit may include an absorber to rid the recirculating gases within the patient circuit of a CO.sub.2 to prevent a CO.sub.2 build-up, various check valves that insure the flow of gas within the patient circuit is in the proper direction, and also a pressure relief valve that vents the patient breathing circuit when the pressure within the circuit reaches a predetermined point so that the patient is not subjected to any excessive pressure. In its use, the pressure relief valve is referred to as an adjustable pressure limiting (APL) valve and is adjustable by the user so that differing maximum pressures are allowed in the patient breathing system during an operation and can be determined by the user.
In conventional valves, the APL valve comprises a simple movable valve member that is seated on a valve seat and which is held against that seat by means of an adjustable force, typical of which is a spring force that is adjustable and which acts against the motion of the movable valve member toward its open position. Thus, by simple adjustment of the spring force acting on the movable valve member, the user can select and change the force required to open that valve and, therefore, adjust the pressure required within the patient breathing circuit to open the valve to vent the patient breathing circuit.
One problem with such valves, however, is that the use of such springs to control a force against the opening of the movable member is that the spring exerts a linear, or near linear, characteristic as the spring is adjusted to change that force. That is, as the user normally rotates the control knob that is threaded to the valve base, the movement of the rotating control knob moves one end of a spring so that the spring is lengthened or shortened to alter the force of the spring against the movable valve member and the opening pressure is thus adjusted. Since it is nearly a linear relationship, a certain rotation of the control knob makes a finite, known change in the spring force generally throughout the range of travel of the rotatable knob.
Accordingly, there is a relatively uniform adjustment of the spring force with movement of the rotatable knob but in the use of the APL valve, it is desirable that the pressure adjustment of the valve be more sensitive at the lower ranges of pressures than at the higher range and it would therefore be advantageous that the valve have a more precise control of the spring bias at those lower pressures and a lesser degree of control at the higher pressures.